Dynamoelectric machines are machines that generate electric power or use electric power. Common types of dynamoelectric machines are alternators, generators, and electric motors.
Electric motors are used in a wide variety of applications involving power tools such as drills, saws, sanding and grinding devices, and yard tools such as edgers and trimmers, just to name a few such tools. These devices all make use of electric motors having an armature and a field, such as a stator.
FIG. 1 shows a typical prior art stator 100 for an electric motor. Stator 100 is formed from a lamination stack 102 around which a plurality of windings of magnet wires 104 are wound to form field coils 114. Lamination stack 102 is formed by stacking together an appropriate number of individual laminations 108 and welding them together. The individual laminations 108 are typically made by stamping them from steel. To do so, loose laminations 108 are loaded in a stacker. The stacker picks up the appropriate number of laminations 108 and places them in a fixture where they are welded together. The laminations 108 are formed with slots so the resulting lamination stack 102 has slots 110 therein in which the magnet wires 104 are wound. Magnet wires, as that term is commonly understood, are wires of the type conventionally used to wind coils in electric machines, such as armatures and stators. Prior to winding the magnet wires 104, insulating sleeves or insulating slot liners (not shown), such as vulcanized fiber, are placed in the slots 110 and end rings 112 placed on the lamination stack 102. End rings 112 are illustratively made of plastic and formed to include coil forms 116. Field coils 114 are then wound by winding the magnet wires 104 in the slots 110. After the field coils 114 are wound, the end of the magnet wires 104 are appropriately terminated, such as to terminals 118 in a terminal post 120. The magnet wires 104 are then bonded together, such as by the application of heat when bondable magnet wires are used. Bondable magnet wires are magnet wires layered with a heat activated thermoplastic or thermoset polymer adhesive. One type of bondable magnet wires commonly used is wire available under the trade name BONDEZE from Phelps Dodge of Fort Wayne, Ind. Alternatively, the magnet wires 104 may be bonded by a trickle resin process described below. Where the stator 100 will be used in an application that exposes it to a particularly abrasive environment, such as a grinder, an epoxy coating is applied to the field coils 114 for abrasion protection.
There are a number of problem areas in the process just described. First of all, it is a capital intensive process. To tool a line to make a stator for a fractional horsepower motor that has a six second cycle time typically requires an investment in the three to five million dollar range. The insulating slot liners must be positioned correctly to meet U.L. (Underwriters Laboratories) requirements and kept positioned properly. In the existing process, the paper slot liners can move when the stator moves to the next station in the process.
The end ring limits slot fill. Slot fill is the amount of magnet wires that can be placed in the slots. The greater the slot fill, the higher the magnetic field generated by the stator. However, increasing the amount of magnet wires placed in the slots can cause the end ring to deform. The end ring can be thickened to reinforce it, but this reduces the slot volume available for the magnet wires.
In the manufacturing process for the stator described above, once the magnet wires have been wound in the slots and the ends of the magnet wires terminated, the magnet wires are bonded if bondable wire is being used and a “trickle” resin is applied over the magnet wires if trickle resin is being used. The process of applying the trickle resin is a somewhat difficult process to manage to obtain consistent results. It also has a number of drawbacks, not the least of which is the cost and difficulty of performing it with reliable, consistent results.
Initially, the trickle process requires the use of a relatively large and expensive oven to carefully preheat the partially assembled stators to relatively precise temperatures before the trickle resin can be applied. The temperature of the trickle resin also needs to be carefully controlled to achieve satisfactory flow of the resin through the slots in the lamination stack. It has proven to be extremely difficult to achieve consistent, complete flow of the trickle resin through the slots in the lamination stack. As such, it is difficult to achieve good flow between the magnet wires with the trickle resin. A cooling period must then be allowed during which air is typically forced over the stators to cool them before the next manufacturing step is taken. Further complicating the manufacturing process is that the trickle resin typically has a short shelf life, and therefore must be used within a relatively short period of time. This requires that batches of the trickle resin be mixed frequently with any that isn't used within its shelf life wasted.
The end result is that stators must often be designed for the process as opposed to optimum performance and cost.